获取C ++中返回值的类型 [英] Get the type of the return value in C++

问题描述

假设我们有一个函数 f ，该函数返回某个未知类型的值（我们称它为 T ），接受 T 类型的值作为参数（可能还有其他一些参数）。如何在C ++ 14中获得 f 的返回类型？

Suppose we have a function f which returns a value of some unknown type (let's call it T) and takes a value of the type T as an argument (and possibly has some other arguments). How do I get the return type of f in C++14?

有一种方法可以做到如果我们知道参数类型（通过 std :: result_of ）。是否可以知道除 T 以外的所有参数类型？

There is a way to do it if we know the know the argument types (via std::result_of). Is it possible if we know all the argument types except T?

示例：

template <class F> // F is functor with   T operator()(T a, T b)
class A {
    // Here I want to do
    // T some_function(T some_arg) { ... }
}

推荐答案

template <typename T>
struct return_type;

template <typename R, typename... Args>
struct return_type<R(Args...)> { using type = R; };

template <typename R, typename... Args>
struct return_type<R(*)(Args...)> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...)> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) &> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) &&> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) const> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) const&> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) const&&> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) volatile> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) volatile&> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) volatile&&> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) const volatile> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) const volatile&> { using type = R; };

template <typename R, typename C, typename... Args>
struct return_type<R(C::*)(Args...) const volatile&&> { using type = R; };

template <typename T>
using return_type_t = typename return_type<T>::type;

测试：

#include <type_traits>

struct Functor
{
    int operator()(int i, int j) { return i + j; }
};

template <class F>
struct A
{
    using T = return_type_t<decltype(&F::operator())>;

    T some_function(T some_arg) { return some_arg; }
};

int main()
{
    A<Functor> a;
    static_assert(std::is_same<decltype(a.some_function(1)), int>::value, "!");
}

演示

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